1. Field of the Invention
This invention relates to a reverse cycle refrigeration system that may operate as an automotive heater or air conditioner. More particularly, the invention pertains to such a system wherein the waste heat from the primary engine coolant and from the engine exhaust gas system are used as heat sources during the heat pump operation.
2. Description of the Prior Art
The heat pump is a device which uses heat energy absorbed by a refrigerant at a low energy state and delivered at high temperature to heat a space after having mechanical work applied. It is known from the prior art that an air conditioning system can be reversed in operation for heating rather than cooling. U.S. Pat. Nos. 3,171,474, 3,141,498, 2,585,748 and 2,806,674 describe the use of an air conditioning system as a reversible heat pump for the purposes of cooling or heating the passenger compartment of a motor vehicle. A recognized difficulty with a reverse cycle system is the tendency to accumulate frost on the evaporator coils during certain periods when atmospheric conditions enhance the possibility of frost accumulation. The prior art has been concerned with adapting heat pump systems to prevent accumulation of frost by various means. U.S. Pat. Nos. 2,988,896, 3,444,699, 3,283,809 and 4,102,391 employ various means either for preventing the accumulation of frost, sensing its presence or dissipating the frost.
Except in regions of mild winter temperature, the outside winter air is not in general a sufficient, low temperature source of heat for a heat pump system. In addition to the operating cost for power and maintenance, there are fixed costs for the equipment of the heat pump, which substantially exceed similar costs for a conventional heating system. Consequently, it is difficult to justify these unless there is need of using the same equipment for cooling in summer.
In conventional practice the passenger compartment of a motor vehicle is heated by heat taken from the primary engine coolant in a radiator. Within the compartment, room temperature air is forced through the heater to enhance the efficiency of the heat exchange. It is recognized that small passenger vehicles, particularly those having efficient engines adapted to deliver high gas mileage, wherein the exhaust gas is recirculated through the engine have an insufficient quantity of waste heat from the coolant to satisfy the requirements for passenger comfort. For example, it has been estimated that perhaps only one-third of the heat load required to maintain the compartment at an average 70.degree. Farenheit temperature can be supplied from the primary coolant of an engine having the efficiency required in future vehicles.